Discharge lamp lighting device and projector

ABSTRACT

A high pressure discharge lamp lighting device includes: a DC power supply circuit that is supplied with a DC voltage and performs current control to supply a predetermined constant power to a high pressure discharge lamp; an inverter that converts an output current of the DC power supply circuit into an AC current having a predetermined frequency and supplies a driving current to the high pressure discharge lamp; and a control unit that controls the DC power supply circuit and the inverter. In the high pressure discharge lamp lighting device, the control unit superposes a current pulse asynchronous with the driving current on the driving current.

BACKGROUND

1. Technical Field

The present invention relates to a high pressure discharge lamp lightingdevice and a projector including the same, and more specifically, to atechnique for preventing flicker.

2. Related Art

In the related art, for example, the following method is used in a highpressure discharge lamp lighting device used for a light source of aprojector in order to stabilize the discharge arc in a high pressuredischarge lamp (hereinafter, referred as a lamp): when an AC lampcurrent is supplied to a high pressure discharge lamp to turn on thehigh pressure discharge lamp, a current pulse having a period obtainedby dividing half the period of the lamp current by a predetermined valueis generated, the polarity of the current pulse becomes identical to thepolarity of the lamp current, and a latter part of the half period ofthe current pulse is superposed on the lamp current (for example,JP-T-10-501919 (see claims and FIG. 4).

In the related art, the current pulse is superposed on the AC lampcurrent in synchronization with a latter part of the half period of theAC lamp current. Therefore, temperature increases in the electrodes inthe lamp, and a discharge arc stably occurs, which prevents flicker.When the lamp is used as, for example, a light source of a projector,the brightness on a screen increases for a time corresponding to thesuperposition of the current pulse on the lamp current, which is anadverse effect. Since the current pulse is synchronously superposed onthe lamp current, a partially bright portion is fixed at a predeterminedposition on the screen, which causes problems that the brightness on thescreen is not uniform and a stripe occurs on the display screen.

SUMMARY

An advantage of some aspects of the invention is that it provides a highpressure discharge lamp lighting device capable of preventing flickerand preventing a stripe from occurring on a display screen and aprojector including the same.

According to an aspect of the invention, a high pressure discharge lamplighting device includes: a DC power supply circuit that is suppliedwith a DC voltage and performs current control to supply a predeterminedconstant power to a high pressure discharge lamp; an inverter thatconverts an output current of the DC power supply circuit into an ACcurrent having a predetermined frequency and supplies a driving currentto the high pressure discharge lamp; and a control unit that controlsthe DC power supply circuit and the inverter. In the high pressuredischarge lamp lighting device, the control unit superposes a currentpulse asynchronous with the driving current on the driving current. Inthis aspect, the current pulse is superposed on the driving current inorder to stabilize the discharge arc occurring in the high pressuredischarge lamp, which makes it possible to prevent flicker caused by avariation in the shape of a leading end of the electrodes in the highpressure discharge lamp. In addition, the superposed current pulse isasynchronous with the driving current, and the relationship between thecurrent pulse and the driving current is random. Therefore, even whenthe high pressure discharge lamp lighting device is used for aprojector, the current pulse causes a corresponding portion of thedisplay screen to be bright, but the bright portion is not fixed, whichmakes it difficult for a viewer to view the bright portion. As a result,the viewer cannot recognize the bright portion as a stripe on thedisplay screen.

In the high pressure discharge lamp lighting device according to thisaspect, preferably, the control unit adjusts the width and/or height ofthe current pulse according to the power supplied to the high pressuredischarge lamp. According to the above-mentioned structure, the controlunit performs constant power control on the high pressure discharge lampand adjusts the width and/or height of the current pulse according tothe power. Therefore, it is possible to stably generate an AC arc andprevent an excessively large amount of energy from being supplied to theelectrodes in the high pressure discharge lamp. As a result, it ispossible to prevent the electrodes in the high pressure discharge lampfrom wearing off.

In the high pressure discharge lamp lighting device according to theabove-mentioned aspect, preferably, the control unit includes afrequency regulator for setting the frequency of the current pulse, andthe control unit set the frequency of the current pulse on the basis ofthe frequency set by the frequency regulator. According to theabove-mentioned structure, the control unit sets the frequency of thecurrent pulse on the basis of the frequency set by the frequencymodulator. Therefore, the control unit can arbitrarily set the frequencyof the current pulse.

In the high pressure discharge lamp lighting device according to theabove-mentioned aspect, preferably, the control unit superposes thecurrent pulse on the second half of the pulse width of the drivingcurrent. According to the above-mentioned structure, the current pulseis superposed on the second half of the pulse width of the drivingcurrent, which makes it possible to effectively stabilize the dischargearc in the high pressure discharge lamp and to prevent flicker.

In the high pressure discharge lamp lighting device according to theabove-mentioned structure, preferably, the control unit randomlysuperposes the current pulse on the second half of the pulse width ofthe driving current. According to the above-mentioned structure, sincethe current pulse is superposed on the second half of the pulse width ofthe driving current it is possible to effectively stabilize thedischarge arc in the high pressure discharge lamp. Even when the highpressure discharge lamp lighting device is used for a projector, theposition of the current pulse is not fixed, which makes it possible toprevent flicker on the display screen.

In the high pressure discharge lamp lighting device according to theabove-mentioned aspect, preferably, the control unit generates areference pulse for generating the current pulse, blocks a portion ofthe reference pulse corresponding to the first half of the pulse widthof the driving current, and superposes the current pulse on the secondhalf of the pulse width of the driving current on the basis of the otherportions of the reference pulse that are not blocked.

According to another aspect of the invention, a projector includes: ahigh pressure discharge lamp; the high pressure discharge lamp lightingdevice according to the above-mentioned aspect; liquid crystal displaypanels; an optical system that guides light emitted from the highpressure discharge lamp to the liquid crystal display panels; and aprojecting unit that projects images displayed on the liquid crystaldisplay panels onto a screen. In the projector, the frequency of acurrent pulse of the high pressure discharge lamp lighting device isdifferent from the frequencies of vertical synchronization signals ofthe liquid crystal display panels. According to this aspect of theinvention, the frequency of the current pulse of the high pressuredischarge lamp lighting device is different from the frequencies of thevertical synchronization signals of the liquid crystal display panels,and the current pulse causes a corresponding portion on the displayscreen to be bright. However, the current pulse is inserted into adifferent position of each frame of the display screen, and the brightposition is not fixed, which makes it difficult for a viewer to view thebright portion. As a result, the viewer cannot recognize the brightportion as a stripe on the display screen.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers refer like elements.

FIG. 1 is a block diagram illustrating the structure of a high pressuredischarge lamp lighting device according to a first embodiment of theinvention.

FIGS. 2A-F are timing charts illustrating signal waveforms forgenerating a lamp current shown in FIG. 1.

FIG. 3 is a drawing illustrating an example of a display screen when thehigh pressure discharge lamp lighting device shown in FIG. 1 is used asa light source of a projector.

FIGS. 4A-F are timing charts illustrating signal waveforms forgenerating the lamp current shown in FIG. 1.

FIG. 5 is a diagram illustrating the structure of an optical system of aprojector incorporating the high pressure discharge lamp lighting deviceaccording to the first embodiment with an illumination optical system.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

FIG. 1 is a block diagram illustrating the structure of a high pressuredischarge lamp limiting device according to a first embodiment of theinvention. The high pressure discharge lamp lighting device shown inFIG. 1 includes a down chopper 11, an inverter 12, an igniter 13, aDC/DC converter 14, and a control circuit 15. A lamp 20 is connected tooutput terminals of the igniter 13. The down chopper 11 corresponds to aDC power supply circuit according to the embodiment of the invention andadjusts an input DC voltage to supply a predetermined constant power tothe lamp 20. In this embodiment, the down chopper 11 performs a chopperprocess on an input voltage to drop the input voltage and performscurrent control to supply a predetermined constant power to the lamp 20.A current output from the down chopper 11 is input to the inverter 12.Resistors R1 and R2 are connected in parallel to output terminals of thedown chopper 11, and the potential of a node between the resistors R1and R2 is supplied to the control circuit 15 as an output voltage of thedown chopper 11. A resistor R3 is connected in series to a negativepotential side of the down chopper 11. A current flowing through theresistor R3 is detected as a lamp current (driving current) and issupplied to the control circuit 15.

The inverter 12 is composed of, for example, four switching elementsconnected to one another in a full bridge manner. The inverter 12alternately switches the four switching elements to convert an input DCvoltage to an AC voltage, and the converted AC voltage is output to theigniter 13. The igniter 13 includes, for example, an igniter transformerand a circuit for driving the igniter transformer, and generates a highvoltage when turning on the lamp 20 and applies the high voltage to thelamp 20. The DC/DC converter 14 generates a driving voltage for thecontrol circuit 15. The DC/DC converter 14 drops an input voltage andsupplies the dropped voltage to the control circuit 15. The controlcircuit 15 is composed of, for example, a microprocessor, and controlsthe down chopper 11, the inverter 12, and the igniter 13. The controlcircuit 15 is supplied with an output voltage and current from the downchopper 11 and controls an output current of the down chopper 11 suchthat constant power is supplied to the lamp 20. In addition, the controlcircuit 15 appropriately controls an output frequency of the inverter 12and controls the igniter 13 to generate a high voltage when turning onthe lamp 20. An external control IF 15 a for receiving control signalsfrom the outside and a variable resistor VR are connected to the controlcircuit 15. The variable resistor VR corresponds to a frequencyregulator used in the invention, and the variable resistor VR isadjusted to regulate the frequency of current pulses (which will bedescribed later) superposed on the lamp current. The lamp 20 is, forexample, a reflective light source device. In the lamp 20, alight-emitting tube 21 is fixed to the center of a reflecting mirror 22with a heat-resistant cement interposed therebetween.

Next, the operation of the high pressure discharge lamp lighting deviceshown in FIG. 1 will be described below. The down chopper 11 performs achopper process on an input DC voltage to drop the input DC voltage andoutputs a current corresponding to the dropped DC voltage to theinverter 12. The inverter 12 converts the input DC current into an ACcurrent having a predetermined frequency and outputs the AC current tothe igniter 13. The igniter 13 generates a high voltage when the lamp 20starts to be turned on and supplies the high voltage to the lamp 20.When the lamp 20 is turned on, the output voltage of the inverter 12 isapplied to the lamp 20, which causes the lamp to be maintained in the onstate. In this case, the control circuit 15 is supplied with the outputvoltage and output current of the down chopper 11 and controls the downchopper 11 such that constant power is supplied to the lamp 20. However,in this embodiment, in order to stabilize a discharge arc occurring inthe lamp 20, the control circuit 15 controls the down chopper 11 suchthat a current pulse is superposed on a lamp current supplied to thelamp 20. A detailed description thereof will be made with reference toFIG. 2.

FIG. 2 is a timing chart illustrating signal waveforms for generatingthe lamp current shown in FIG. 1. In FIG. 2, the character A indicates areference pulse string, and the character B indicates a lamp lightingwaveform corresponding to a lamp current (a square-wave current flowingfrom the inverter to the lamp) without a current pulse string. Thecharacter C indicates a pulse string superposed on one polarity of thelamp lighting waveform B, and the character D is a pulse stringsuperposed on the other polarity of the lamp lighting waveform B. Sincethe polarity of the pulse string D is opposite to the polarity of thepulse string C, the pulse string D is inverted for use. The character Findicates a waveform obtained by superposing the pulses C and E on thelamp lighting waveform B, and a lamp current corresponding to the lampcurrent waveform F is supplied to the lamp 20. When the high pressuredischarge lamp lighting device is applied to a display apparatus, suchas a projector, the frequency of the lamp lighting waveform B is set soas to correspond to the frequency (for example, 50 Hz, 60 Hz, or 72 Hz)of a vertical synchronization signal of a display screen (for example,90 Hz), in order to prolong the life span of the lamp 20. In thisembodiment, the frequency of the reference pulse string A is differentfrom the frequency of the lamp lighting waveform B, and the referencepulse string A is set so as not to synchronize with the lamp lightingwaveform B. That is, when the pulse strings C and E are superposed onthe lamp lighting waveform B to generate the lamp lighting waveform F,the pulse strings C and E are not fixed to a portion of the lamplighting waveform F. Therefore, the frequency of the reference pulsestring A is set in the range of 74 Hz to 88 Hz. The superposition of thepulse strings C and E causes the brightness of the lamp 20 to be high atthat time, but the pulse strings C and E are not fixed to a portion ofthe lamp lighting waveform F. Therefore, when the high pressuredischarge lamp lighting device is used as a light source of a projector,a bright position of the projector is not fixed, but is moved, whichmakes it difficult for a viewer to recognize the bright position on theentire display screen. As a result; the viewer does not care about thebrightness on the display screen.

Therefore, as described above, in order to superpose current pulses toobtain a lamp current corresponding to the lamp lighting waveform F, thecontrol circuit 15 makes a control instruction value when controllingthe output current of the down chopper 11 have the lamp lightingwaveform F, or reduces the feedback amount of the current output fromthe down chopper 11 by the superposed current pulse.

The control circuit 15 changes the width and height of the superposedcurrent pulse of the lamp lighting pulse F on the basis of the powersupplied to the lamp 20. For example, the width Iw2 and the height Ip2of the superposed current pulse are set to about 5% of the width Iw1 andthe height Ip1 of a portion of the lamp power, which is a referencevalue, corresponding to the lamp lighting waveform B, respectively, andare appropriately changed according to a variation in the lamp power.When the width and height of the current pulse are excessively small, anunstable AC arc is generated. Therefore, in order to generate a stableAC arc, the width and/or height of the current pulse are adjusted tohave suitable values and the adjusted values are supplied. When thecurrent pulse has excessively large width and height, an excessivelylarge amount of energy is supplied to the electrodes in the lamp 20, andthe electrodes wear off. In order to prevent the electrodes from wearingoff, the width and height of the current pulse are adjusted to havesuitable values and the adjusted values are supplied. In addition, thefrequency of the reference pulse string A can be adjusted to anarbitrary value by adjusting the resistance of the variable resistor VR.For example, it is possible to adjust the resistance of the variableresistor VR while viewing the display screen for a projector.

FIG. 3 is a drawing illustrating an example of a display screen when thehigh pressure discharge lamp lighting device shown in FIG. 1 is used asa light source of a projector. As shown in FIG. 3, a sparse belt (brightportion) like fog is not fixed at a predetermined position, but isrepeatedly moved in the vertical direction, which makes it difficult fora viewer to view the belt. As a result, it seems that no stripe occurson the display screen (the bright portion like belt is not recognized asa stripe), and thus the viewer is not bothered by the stripe on thedisplay screen. In the related art (JP-T-10-501919), since the currentpulse string synchronizes with the lamp lighting waveform, the sparsebelt shown in FIG. 3 is displayed at a fixed position on the screen anddisplayed as a stripe at a predetermined position, and thus the viewercan not view the screen cleanly.

In the above-described first embodiment, the current pulse is superposedon the lamp current in order to stabilize the discharge arc occurring inthe lamp 20, which makes it possible to prevent flicker caused by thedeterioration of the quality of the lamp 20 over time. In addition, thesuperposed current pulse does not synchronize with the lamp current, andthe relationship between the current pulse and the lamp current israndom. Even when the high pressure discharge lamp lighting device isused for, for example, a projector, the superposed current pulse causesa corresponding portion of the display screen to be bright, but thebright position is not fixed, which makes it difficult for a viewer torecognize the bright portion. As a result, a stripe does not appear onthe display screen.

Second Embodiment

FIG. 4 is a timing chart illustrating another example of the signalwaveforms for generating the lamp current shown in FIG. 1. In FIG. 4,the character G indicates a mask pattern. When pulses in pulse strings Cand E are disposed at the first half of the pulse width of a lamplighting waveform B, such pulses are masked by the mask pattern G. Onlywhen pulses in the pulse strings C and E are disposed at the second halfof the pulse width of the lamp lighting waveform B, such pulses aresuperposed on the lamp lighting waveform B. In this way, the pulsestrings C and E are superposed on the second half of the pulse width ofthe lamp lighting waveform B to generate a lamp lighting waveform F.Since the position of the superposed pulse string is not fixed, the sameeffect as that in the first embodiment can be obtained. In addition, thecurrent pulse is superposed on the second half of the pulse width of thelamp current, which makes it possible to stabilize the discharge arc ofthe lamp 20 and thus effectively prevent flicker.

Third Embodiment

FIG. 5 is a diagram illustrating the structure of an optical system of aprotector, which is an assembly of the high pressure discharge lamplighting device according to the first or second embodiment and anillustration optical system. A high pressure discharge lamp lightingdevice 10 shown in FIG. 5 corresponds to the high pressure dischargelamp lighting device 10 shown in FIG. 1. In this embodiment, thefrequency of the superposed current pulse differs from the frequency ofthe driving current of the lamp 20 and the frequencies of verticalsynchronization signals of liquid crystal display panels 250, 252, and254.

The projector includes an illustration optical system 100, dichroicmirrors 210 and 212, reflecting mirrors 220, 222, and 224, anincident-side lens 230, a relay lens 232, three field lenses 240, 242,and 244, three liquid crystal display panels 250, 252, and 254,polarizing plates 251, 253, 255, 256, 257, and 258 arranged on theemission sides and the incident sides of the liquid crystal displaypanels, a cross dichroic prism 260, and a projection lens 270.

The illumination optical system 100 includes a high pressure dischargeamp 20 for emitting a substantially parallel light beam, an illuminatingdevice 120, a reflecting mirror 150, and a condenser lens 160. The highpressure discharge lamp 20 is composed of a light-emitting tube 21 (seeFIG. 1), serving as a radial light source for emitting a radial lightbeam. The light beam emitted from the high pressure discharge lamp 20 isincident on the illuminating device 120, and the illuminating device 120emits a light beam having uni form brightness. The light beam isreflected from the reflecting mirror 150 to be incident on the condenserlens 160. The condenser lens 160 makes the light beam having uniformbrightness that is emitted from the illustrating device 120 incident onthe liquid crystal display panels 250, 252, and 254.

The two dichroic mirrors 210 and 212 form a color light separatingoptical system 214 that separates the light emitted from theillumination optical system 100 into three light beams, that is, a red(R) light beam, a green (G) light beam, and a blue (B) light beam. Thefirst dichroic mirror 210 transmits the red light beam of the lightbeams emitted from the illumination optical system 100, but reflects theblue light beam and the green light beams.

The red light beam passing through the first dichroic mirror 210 isreflected from the reflecting mirror 220 to be incident on the redliquid crystal display panel 250 through the field lens 240. The fieldlens 240 has a function of condensing light beams such that the lightbeams are parallel to the main light beam (central axis) of the lightbeams. The other field lenses 242 and 244 provided in front of thecorresponding liquid crystal display panels have the same function asthe field lens 240.

Of the blue light beam and the green light beam reflected from the firstdichroic mirror 210, the green light beam is reflected from the seconddichroic mirror 212 to be incident on the green liquid crystal displaypanel 252 through the field lens 242. Meanwhile, the blue light beampasses through the second dichroic mirror 212 and further passes througha relay lens system including the incident-side lens 230, the relay lens232, and the reflecting mirrors 222 and 224. The blue light beam passingthrough the relay lens system is incident on the blue liquid crystaldisplay panel 254 through the field lens 244.

The three liquid crystal display panels 250, 252, and 254 serve as lightmodulating devices for converting the incident red, green, and bluelight beams into light components for forming an image on the basis ofinput image signals. The polarizing plates 256, 257, and 258 areprovided on the incident sides of the liquid crystal display panels 250,252, and 254, respectively, and the polarizing plates 251, 253, and 255are provided on the emission sides of the liquid crystal display panels250, 252, and 254, respectively. The polarizing plates adjust thepolarizations of the red, green, and blue light beams. The light beamspassing through the liquid crystal display panels 250, 252 and 254 areincident on the cross dichroic prism 260.

The cross dichroic prism 260 serves as a color combining optical systemfor combining the red, green, and blue light beams emitted from thethree liquid crystal display panels 250, 252, and 254. In the crossdichroic prism 260, a dielectric multi-layer film for reflecting the redlight beam and a dielectric multi-layer film for reflecting the bluelight beam are provided at interfaces among four right-angled prisms ina substantially X shape. The dielectric multi-layer films combine threecolor light beams to form a combined light beam for protecting a colorimage. The combined light beam generated by the cross dichroic prism 260is incident on the protection lens 270, and is then projected onto aprojection screen 300. In this way, the images displayed by the liquidcrystal display panels 250, 252, and 254 are projected onto theprojection screen 300.

As described above, in the third embodiment, the frequency of thesuperposed current pulse of the discharge lamp lighting device differsfrom the frequency of the lamp current of the lamp 20 and thefrequencies of the vertical synchronization signals of the liquidcrystal display panels 250, 252, and 254. Therefore, the superposedcurrent pulse causes a corresponding portion of the display screen to bebright, but the bright position is not fixed, which makes it difficultfor a viewer to recognize the bright portion. As a result, a stripe doesnot appear on the display screen.

The entire disclosure of Japanese Patent Application Nos: 2006-33490,filed Feb. 10, 2006 is expressly incorporated by reference herein.

1. A high pressure discharge lamp lighting device comprising: a DC powersupply circuit that is supplied with a DC voltage and performs currentcontrol to supply a predetermined constant power to a high pressuredischarge lamp; an inverter that converts an output current of the DCpower supply circuit into an AC current having a predetermined frequencyand supplies a driving current to the high pressure discharge lamp; anda control unit that controls the DC power supply circuit and theinverter, wherein the control unit superposes a current pulseasynchronous with the driving current on the driving current.
 2. Thehigh pressure discharge lamp lighting device according to claim 1,wherein the control unit adjusts the width and/or height of the currentpulse according to the power supplied to the high pressure dischargelamp.
 3. The high pressure discharge lamp lighting device according toclaim 1, wherein the control unit includes a frequency regulator forsetting the frequency of the current pulse, and the control unit setsthe frequency of the current pulse on the basis of the frequency set bythe frequency regulator.
 4. The high pressure discharge lamp lightingdevice according to claim 1, wherein the control unit superposes thecurrent pulse on the second half of the pulse width of the drivingcurrent.
 5. The high pressure discharge lamp lighting device accordingto claim 4, wherein the control unit randomly superposes the currentpulse on the second half of the pulse width of the driving current. 6.The discharge lamp lighting device according to claim 4, wherein thecontrol unit generates a reference pulse for generating the currentpulse, blocks a portion of the reference pulse corresponding to thefirst half of the pulse width of the driving current, and superposes thecurrent pulse on the second half of the pulse width of the drivingcurrent on the basis of the other portions of the reference pulse thatare not blocked.
 7. A projector comprising: a high pressure dischargelamp; the high pressure discharge lamp lighting device according toclaim 1; liquid crystal display panels; an optical system that guideslight emitted from the high pressure discharge lamp to the liquidcrystal display panels; and a projecting unit that projects imagesdisplayed on the liquid crystal display panels onto a screen, whereinthe frequency of a current pulse of the high pressure discharge lamplighting device is different from the frequencies of verticalsynchronization signals of the liquid crystal display panels.